1. Field of the Invention
The present invention relates in general to digitally controlled oscillators. More particularly, the present invention relates to an offset (non-symmetrical) digitally controlled oscillator which is capable of functioning in a linear mode and is especially desirable for use in phase locked loop circuitry.
2. Description of the Prior Art
Various configurations of circuitry have previously been used in phase locked loop circuitry to generate the necessary output frequency. The first and most common configuration is the voltage controlled oscillator (VCO) as used in the analog phase locked loop. The input signal to the VCO is a time varying voltage and is converted by the VCO to a proportional frequency. The output of the VCO is fed back to an appropriate point as a reference signal. A typical VCO in this type of configuration includes a varactor, the impedance of the feedback network is changed and thus the resonant frequency of the oscillator is changed. The use of the varactor introduces slight non-linearity to the transfer function of the VCO.
Various other configurations employ the use of a symmetrical digitally controlled oscillator (DCO). In prior versions of DCO circuit implementation, frequency generation has been accomplished through the use of a divide by n, n-1 or n+1 symmetrical configuration. In this type of circuit, the center frequency is:
f.sub.c =f.sub.HS /n, where n is the high speed divide ratio. Frequencies greater than f.sub.c are generated by dividing f.sub.HS by n-1 and frequencies less than f.sub.c are generated by dividing f.sub.HS by n+1. In order to linearize the transfer function of this configuration, a ROM or other programmable device was used to distort the input to the DCO.
The present invention as claimed is intended to provide an offset digitally controlled oscillator which eliminates many of the prior art deficiencies which include the need to introduce predistortion into the input signal to the DCO. Many of the prior art implementations are not sufficiently reliable and are not sufficiently repeatable regarding their operation.